<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>

  <!-- 1. script标签不能使用单标签 即使是外链的js文件也一样-->
  <!-- 2. script标签应该写在body子元素的最后一行 -->
  <script></script>
</body>
</html>